In recent years, an examination for applying a high-strength steel sheet to the vehicle body has been actively made to reduce the weight of a vehicle body from the viewpoint of global environment protection, and thus strength demanded for a steel material has been increasing. However, workability of a steel sheet deteriorates as the strength of the steel sheet increases, and thus the shape-freezing properties need to be considered.
On the other hand, in commonly used press working, a forming load gradually increases, and thus there is a significant problem with improvement in pressing capability in terms of being put into practical use.
In a hot stamping technology, press forming is carried out after heating a steel sheet to a high temperature of an austenite range. Accordingly, the forming load is greatly reduced compared to common press working that is carried out at room temperature.
In addition, in the hot stamping technology, a hardening treatment is carried out concurrently with the press working by cooling the steel sheet in a die, and thus strength corresponding to the content of C in steel may be obtained. Accordingly, the hot stamping technology has attracted attention as a technology of making the shape freezing properties and the strength compatible with each other.
Patent Document 1 discloses a method of obtaining a hot stamped article having tensile strength of 980 MPa or more as a hot stamping technology. However, in this method, it is difficult to obtain a hot stamped article having tensile strength lower than 980 MPa.
Patent Document 2 and Patent Document 3 disclose a technology related to a member using a hot stamping material with low tensile strength, and a production method thereof, and a technology related to a member by a tailored blank to which the technology is applied. However, in these technologies, consideration is not made for delayed fracture characteristics and toughness, and thus it is difficult to say that performance as a member is sufficient.